The Apicomplexan protozoa are a phylum of diverse obligate intracellular parasites. Infection by Apicomplexan parasites causes incalculable morbidity and mortality to mammals such as humans and agricultural animals. Apicomplexan species, such as Plasmodium, Babesia, Cryptosporidium, Isospora, Cyclospora, Sarcocystis, and Toxoplasma, produce disease of varying severity.
Toxoplasma is responsible for diseases throughout the world (except extremely cold or dry climates) and tends to be more prevalent in tropical climates. Serologic studies have shown prevalence rates up to 70% by the age of 25 in some central American populations. In the United States an estimated 0.5-1% of the population becomes infected each year and prevalence ranges from 10-25% by the age of 25.
Although toxoplasmosis is most often a benign disease, noted exceptions are in the cases of congenital infection or immunocompromised individuals. Congenital toxoplasmosis is particularly severe and occurs in one of every one thousand births. Infection can result in spontaneous abortion, premature birth, or full-term with or without progressive disease. Typical congenital toxoplasmosis disease manifestations include retinochoroiditis, intracerebral calcification, hydrocephaly, microcephaly, and psychomotor disturbances. Clinical outcomes indicate 5-10% death, 8-10% severe brain or eye damage, 10-13% moderate-to-severe visual handicaps, 58-72% asymptomatic at birth, developing, retinochoroiditis or mental handicaps later in life.
Toxoplasmosis has been long noted as an opportunistic infection in regards to reactivation of latent infections due to immunosuppression associated with organ transplants and certain cancer treatments. During the 1980s toxoplasmic encephalitis emerged as a common complication associated with AIDS. Early symptoms of toxoplasmic encephalitis can include headache, fever, lethargy, and altered mental status with progression to focal neurological deficits and convulsions. The disease is almost always due to a reactivation of a latent infection and tends to remain confined to the central nervous system. The focal lesions are caused by the destruction of host cells in the immediate vicinity. Other forms of the reactivated disease, especially retinochoroiditis, pneumonitis, myocarditis and myositis, may occasionally occur in conjunction with immunosuppression. Toxoplasma gondii is a significant opportunistic pathogen in immunocompromised individuals such as those infected by HIV-AIDS.
Eimeria, pathogens of chicken and cattle, Theileria, tick-borne parasites of cattle in Africa, and Cryptosporidium, an animal parasite as well as an opportunistic pathogen of humans, are members of the phylum. Babesiosis is a rare zoonotic infection transmitted by ticks. The etiological agents, Babesia species, are blood parasites which infect a wide variety of wild and domestic animals throughout the world. Babesia and Theileria form a group called the piroplasms, in reference to intraerythrocytic forms that are pear-shaped in some species. Piroplasms cause tremendous losses of livestock in endemic areas.
Plasmodium is the causative agent in malaria. Malaria is the most common and deadly parasitic disease in the world. In any given year, there will be 300-500 million cases of malaria and 1-3 million human fatalities, and about 400 million people are infected. (Trigg, P. I., and A. V. Kondrachine (1998) The Current Global Malaria Situation, Chapter 2, p. 11-22, in MALARIA PARASITE BIOLOGY, PATHOGENESIS AND PROTECTION. Ed. I. W. Sherman, ASM Press, Washington, D.C.) The global situation continues to worsen due to the lack of availability of effective drugs, because there is no effective vaccine for malaria, and because the responsible parasite Plasmodium has rapidly developed resistance to all currently available drugs. In an increasingly wide geographic area, both Plasmodium falciparum and Plasmodium vivax have been developing resistance to chloroquine, the most successful antimalarial drug in the past several decades. Mefloquine and doxycycline, the two other frontline drugs for the treatment and prevention of malaria are becoming increasingly ineffective. (See Vroman, J. A. et al. (1999) Curr. Pharm. Design 5:101-138.) Artemisinin analogs such as artesunate and arteether were later introduced that are found to be quite effective, particularly against drug-resistant P. falciparum but observations of drug-induced and dose-related neurotoxicity in animals have raised concern about the safety of these compounds for human use. (See Bhattacharjee, A. K. and J. M. Karle (1999) Chem. Res. Toxicol. 12: 422-428 and Genovese, R. F. and D. B. Newman (2008) Arch. Toxicol. 82 (6):379-85.)
Several protozoa parasites are listed as Category B biodefense targets by the National Institute for Allergy and Infectious Diseases (NIAID), including Cryptosporidium parvum, Cyclospora cayatanensis, and Toxoplasma. Cyclospora cayatanensis and Cryptosporidium parvum cause severe persistent diarrhea. The phylum also includes gregarines, parasites of the guts of invertebrates including cockroaches and shrimp. These protozoan parasites share distinctive morphological features, cytoskeletal organization, and modes of replication, motility, and invasion.
Infection by Apicomplexan parasites causes serious infectionsin healthy people, but are extremely dangerous and often deadly in immunocompromised individuals (e.g., HIV/AIDS patients, organ recipients, people undergoing chemotherapy), in children and the elderly, and in pregnant women. They also pose imminent threats to homeland security as agents of bioterrorism, since infections are readily spread via contaminated food and water. Vaccines are non-existent, and adequate drug treatment is lacking due to acquired drug resistance by these parasites and severe adverse reactions by patients to existing treatments (e.g., sulfa drugs). For these reasons, these infections are designated as “Biowarfare Pathogens” by the National Institutes of Health (NIH).
Toxoplasmosis is the leading cause of severe congenital neurological defects (brain and eye damage) in humans, and is a frequent cause of still births and spontaneous abortions. Toxoplasmosis is also a very serious opportunistic infection that causes blindness and life-threatening meningitis in immuno-compromised individuals. Toxoplasmosis is still an important opportunistic infection of the central nervous system in patients infected with HIV/AIDS. (Hoffmann, C., et al. (2007) Clin. Microbial Infect. 13:510-515.) Approximately 10% of AIDS patients in the USA and up to 30% in Europe were estimated to die from toxoplasmosis in 2005 (Davaro, R. E. and A. Thirumalai (2007) J. Intensive Care Med. 22 (2):73-81.)
Since the introduction of highly active antiretroviral therapy (HAART), the rates of opportunistic infections have dropped markedly as has overall morbidity and mortality from HW infection in developed countries. However, opportunistic infections remain the most important cause of death in HIV-infected people due to both late presentation of HIV infections and failure of HAART to adequately restore cell-mediated immunity in all individuals. Persistent neurological deficits are often present in surviving patients (Hoffmann, C., et al. (2007) Clin. Microbial Infect. 13:510-515) and lifelong maintenance therapy is needed to prevent recurrent opportunistic infection (Manzardo, C., et al. (2005) J. Neurovirol. 11 Suppl 3:72-82.) For those patients who fail HAART, those who are unable to tolerate it, or those whose treatments are interrupted, therapy against opportunistic infections remains essential.
Reemergence of toxoplasmosis as a life-threatening disease in HIV/AIDS patients is anticipated in the wake of emerging multi-drug resistant strains of HW (Omrani, A. S. and D. Pillay (2000) Hosp. Med. 61 (5):304-5; Omrani, A. S. and D. Pillay (2000) J. Infect. 41 (1):5-11.) Toxoplasmic encephalitis in those with AIDS in the developing world remains a frequent and devastating problem. Encephalitis is the most serious manifestation of toxoplasmosis in immunosuppressed patients as it causes severe neurologic and rapidly fatal damage to such patients (Hill, D. and J. P. Dubey (2002) Clin. Microbiol. Infect. 8 (10):634-40.) Toxoplasmic chorioretinitis is less frequent than toxoplasmic encephalitis in patients with AIDS but both have serious morbidity. Cerebral toxoplasmosis is the most common neurological mass lesion in patients with AIDS (Hoffmann, C., et al. (2007) Clin. Microbial Infect. 13:510-515; Cohen, B. A. (1999) Semin. Neurol. 19 (2):201-11; Simpson, D. M. and M. Tagliati (1994) Ann. Intern. Med. 121 (10):769-85.) Treatments for Toxoplasmosis exist, however their failure to kill the parasitic cysts leads to frequent relapses and their toxicity often forces discontinuation of the drug.
Cryptosporidiosis (Crypto) is a leading cause of waterborne disease in humans worldwide. Crypto causes an astounding 500 million cases of diarrhea annually, which is especially serious and often fatal in immuno-compromised individuals. Crypto-associated diarrhea is also a widespread problem in the cattle industry. At least 45 serious outbreaks of Crypto were reported in the USA during the past 20 years, including a massive outbreak in 1993 when over 300,000 people in Milwaukee became gravely ill and scores died. The Crypto parasite is resistant to disinfectants such as chlorine bleach; thus it can survive in chlorinated swimming pools and water parks. There is no known effective therapy for human Cryptosporidiosis.
Cyclosporiasis, which causes prolonged diarrhea and other gastrointestinal problems in humans, is especially severe and sometimes deadly in immuno-compromised individuals (e.g., HIV/AIDS patients). Although therapeutic treatments currently exist, many of them are contraindicated for pregnant women, the elderly, and in people with hepatic and renal impairment. No alternative drugs have been identified to date for people who are unable to take sulfa drugs.
In view of the serious global threat posed by these Apicomplexan-related diseases, there is an urgent unmet need for safe and effective medicines that are useful for prophylaxis and treatment. There is a further urgent unmet need for pesticides against Apicomplexa parasites. There is still a further unmet urgent need for agents for use against several protozoa parasite biodefense targets listed as Category B agents by the NIAID.